> could we simulate the Universe With what could we possibly simulate all there is?
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Alfred CentauriJul 31 '12 at 20:12

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Hi Event_Horizon, and welcome to Physics Stack Exchange! We prefer that you ask one question per post, so I've taken out your related questions. But feel free to ask additional questions in separate posts.
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David Z♦Jul 31 '12 at 21:00

@DavidZaslavsky Thanks but the questions were relevant to the main question, we would need to know those things to do a proper simulation.
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Event_HorizonAug 1 '12 at 13:42

2 Answers
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As of 20th July Craig Venter's group have just managed to simulate the bacterium with the smallest known genome. This is a very high level simulation starting at the gene level, and it's the simplest known (non-viral) organism. Even so it's just possible. Starting even one layer down, e.g. with molecular dynamics, would make it impossible (at present!).

In any case, there's no point to such a simulation. At any point in a hierarchy we have an effective theory that works very well but is only a low energy approximation. For example quantum mchanics is (probably) a low energy approximation to string theory, but you wouldn't use string theory to calculate the results of the two slit experiment. Well, you could do but you'd get exactly the same answer as if you just used regular QM.

So you wouldn't use string theory to attempt to simulate life because regular QM is simpler and would give the same results. Actually you wouldn't even use QM because organic chemistry is simpler and gives the same results. And so on.

The point is that we can be confident in our approximation using just chemistry because we know it's based on quantum mechnaics. We know we can use regular quantum mechanics because it's based on quantum field theory, and so on.

It is, in fact, logically impossible for any observer to exactly simulate any system in which the observer is included because it would lead to infinite recursion. This is fairly straight-forward, but there is a mathematical proof out there... I honestly wish I remembered where it was because I've been looking for it for a long time.

Even if it were not logically impossible, it is practically impossible for the reasons cited by John and Alfred. Additionally, a simulation is simply a calculation that yeilds an expected value at a later time. However, The universe (as far as we can tell) follows the principle of least action meaning that the most energetically efficient method of calculating the outcome of any particular event (as would be necessary for your simulation) is the actual event itself. Meaning, the most efficient method of calculating the future state of an object subject to specified forces, is to run the experiment and measure the result.

In other words, it takes more energy for Ventner's simulation of a bacterium than the bacterium uses to go through the same motion. Practically speaking, then, the most efficient "simulation" of the universe, is the universe itself.